1. Field of the Invention
The present invention relates to a miniature electric motor (fractional horse-power motor or fractional wattage motor) of a type used to drive an audio or video tape recorder, for example, with a stator comprised of a rubber magnet.
2. Background Information
There are basically two conventional manners of assembling the stators of the conventional miniature electric motor. One method, exemplified in FIG. 4(a), is to arrange two separate pole pieces made of a rubber magnet each having a section of "C" configuration, along an inner circumference of a motor housing. Alternatively, in FIG. 4(b), the stator is structured by forming a circular stator from a rectangular strip of a rubber magnet to conform to the entire curvature of the inner circumference of the motor housing. Once placed inside the housing, the strip is polarized under predetermined conditions.
Of the above-mentioned arrangements, the latter conventional example is the more frequently and widely employed because of the simplicity of the assembly process and low manufacturing cost. The efficiencies of this design, however, are not realized in the case where the whole motor configuration is restricted an elliptical shape or the like.
The stator structured with two pole pieces made of a rubber magnet with sections of "C" configuration is advantageous in that it requires only a small amount of rubber magnet. It however has drawbacks because it entails a complex assembly process which may sometimes require a long time for adhering and fixing the magnet pieces to the inner circumference of the motor housing. Also, the motor may have a high defect rate due to possible separation of the once fixed rubber magnet pieces.
The rectangular rubber magnet strip of the alternative design has the advantage t hat the strip can easily be held in the motor housing and can be assembled in the housing in a short time period because the strip closely contacts the entire inner circumference of the housing. The rectangular rubber magnet strip however has a drawback because the housing must hold a circular long strip of magnet including non-polarized parts which occupy spaces which could otherwise be spared. Another disadvantage is caused by the hardness or brittleness inherent to the rubber magnet material which may sometimes adversely affect the characteristics of the motor itself. The joining edges of the rectangular strip may sometimes lift slightly from the inner circumference of the housing. This defect will cause the motor to be rejected due to the strict standard applied in the manufacturing control of such miniature motors. Strict standards apply; for instance, to the uneven gap or clearance between the stator and the rotor.
In addition, the polarizing process itself also imposes an unsolved problem of designing the shape of a polarizing yoke, because the shape of the polarizing yoke has a serious influence on the maximum torque and the cogging characteristics of the finished motor. This problem would require a long time period to conduct experiments and evaluate the experimental results.
Apart from this, the necessity for reducing the cogging characteristics of such a miniature motor has been recognized and some improvements have recently been developed and disclosed in technical papers. One of such improvements may comprise, for instance, providing comb teeth on salient poles of an armature core which forms a rotor.
In actual practice, forming comb teeth on the salient poles of the armature core of the rotor has been and is now being performed by die punching or etching comb teeth on thin leaves which form the core, so that the comb teeth can reduce the cogging characteristics as much as possible.
The step of laminating the thin leaves to form a core makes it difficult to mass-produce a core of a miniature motor intended to realize a so-called 37 light-weight, thin, short and miniature size" structure. Thus, it is realized that any means other than providing comb teeth on the rotor in this manner would be preferable for effectively improving the cogging characteristics of the motor.
Moreover, the stator of the rectangular rubber magnet strip, which allows for ease of the assembly process has another drawback that the non-polarized regions actually occupy about one half (1/2) of the strip's entire length, being redundant and not necessary to achieve the desired motor characteristics.